1. Field of the Invention
The present invention relates to method and device for lighting high pressure discharge lamps capable of preventing arc jump by superposing pulse current.
2. Description of the Related Art
High pressure discharge lamps such as ultra-high pressure discharge lamps or metal halide lamps are frequently used as light sources in optical information systems such as a liquid crystal projector. Recently, high pressure discharge lamps having a relatively short electrode gap (arc length) are being developed for enhancing the luminance and light utilization efficiency.
However, in a high pressure discharge lamp of such a short arc type, the tip end of an electrode is heated to a high temperature and consumed due to evaporation of tungsten. Therefore, when the lamp is continuously lighted for a long period of time, so-called arc jump (a phenomenon that the luminous point between the electrodes fluctuates) is likely to occur, causing luminance variation (flicker) on a screen. Therefore, in the development of a high pressure discharge lamp of the short arc type, the prevention of arc jump has been a significant technical challenge.
As a measure to solve such a problem, a technique for preventing arc jump by superposing pulse current on rectangular alternating lamp current is disclosed in the gazette of JP-A-10-501919.
However, with this conventional technique, as the continuous use time of the high pressure discharge lamp becomes longer, the electrodes grow, so that the arc length becomes smaller. Therefore, the lamp voltage drops gradually, which makes it impossible to continue the lighting. Further, since the luminous point fluctuates as the electrodes grow, the optimum position relative to an optical component such as a reflector cannot be maintained, which may cause deterioration in luminance.